Crystal structure of trans- [W(CO)4(n2-C2H4)2] and IR 1H NMR studies of the reactions of this and related ethene carbonyl complexes of tungsten(0), [W(CO)(n)(n2-C2H4)6-(n)] (n = 3-5)

Article abstract of DOI:10.1039/a808621d

The structure of trans-[W(CO)₄ (η²-C₂H₄)₂] 1 has been determined by X-ray crystallography at 293 K [orthorhombic, space group Aba2, a = 12.458(3), b = 6.370(2), c = 12.557(3)A, Z = 4, R = 0.022] to confirm that the C=C bonds of the two ethene ligands are mutually staggered while eclipsing the central W(CO)₄ unit. Reversible isomerisation of trans- to cis-[W(CO)₄(η²-C₂H₄)₂] 2 under the action of selective irradiation has been shown to occur in solid argon matrices at 14-16 K. ¹H NMR measurements have also been applied to the identification of not only 1 and 2 but also the labile 1Be complexes [W(CO)₅(η²-C₂H₄)] 3, mer-[W(CO)₃(η²C₂H₄)₃] 4, and fac-[W(CO)₃(η²-C₂H₄)₃] 5 formed by broad-band UV-visible photolysis of ethene-saturated hydrocarbon solutions of 1 or [W(CO)₆] in the temperature range 200-293 K. In these circumstances the mixed ethene carbonyl complexes 2-5 decay thermally with dissociation of an ethene ligand and substitution by CO in reactions which have been monitored by reference to the relevant ¹H NMR signals. Pseudo-first-order decay constants in the range 0.6-12 x 10^-4 s^-1 have thus been estimated with temperature-dependences implying activation energies, E(a), that decrease in the order 3 > 2 > 4 > 5.

Full text of DOI:10.1039/a808621d

View Article Online / Journal Homepage / Table of Contents for this issue
Crystal structure of trans-[W(CO) (g2-C H ) ] and IR and
2 4 2
H NMR studies of the reactions of this and related ethene carbonyl
4
1
complexes of tungsten(0), [W(CO) (g2-C H ) ] (n = 3–5)
n
2 4 6~n
Teresa Szyman ska-Buzar,*a Krystyna Kern,a Anthony J. Downs,*b Timothy M. Greene,b
Leigh J. Morrisb and Simon Parsonsc
a Faculty of Chemistry, University of W roc¡aw, F. Joliot-Curie 14, 50 383 W roc¡aw, Poland
b Inorganic Chemistry L aboratory, University of Oxford, South Parks Road, Oxford,
UK OX1 3QR
c Department of Chemistry, University of Edinburgh, KingÏs Buildings, W est Mains Road,
Edinburgh, UK EH9 3JJ
Received (in Cambridge, UK) 4th November 1998, Accepted 4th February 1999
The structure of trans-[W(CO) (g2-C H ) ] 1 has been determined by X-ray crystallography at 293 K
4
2 4 2
[
orthorhombic, space group Aba2, a \ 12.458(3), b \ 6.370(2), c \ 12.557(3) Ó, Z \ 4, R \ 0.022] to conÐrm that
the CxC bonds of the two ethene ligands are mutually staggered while eclipsing the central W(CO) unit.
4
Reversible isomerisation of trans- to cis-[W(CO) (g2-C H ) ] 2 under the action of selective irradiation has been
4
2 4 2
shown to occur in solid argon matrices at 14È16 K. 1H NMR measurements have also been applied to the
identiÐcation of not only 1 and 2 but also the labile 18e complexes [W(CO) (g2-C H )] 3, mer-[W(CO) (g2-
5
2 4
3
C H ) ] 4, and fac-[W(CO) (g2-C H ) ] 5 formed by broad-band UV-visible photolysis of ethene-saturated
2
4 3
3
2 4 3
hydrocarbon solutions of 1 or [W(CO) ] in the temperature range 200È293 K. In these circumstances the mixed
6
ethene carbonyl complexes 2È5 decay thermally with dissociation of an ethene ligand and substitution by CO in
reactions which have been monitored by reference to the relevant 1H NMR signals. Pseudo-Ðrst-order decay
constants in the range 0.6È12 ] 10~4 s~1 have thus been estimated with temperature-dependences implying
activation energies, E , that decrease in the order 3 [ 2 [ 4 [ 5.
a
There are signiÐcant synthetic, catalytic and theoretical
aspects to the interest recently aroused by the alkene complex-
es derived from photochemical reactions of transition-metal
carbonyls.1h19 We have sought systematically to explore the
W(CO) fragment.
4
IR studies of the photolysis of trans-[W(CO) (g2-alkene) ]
(alkene \ prop-1-ene, but-1-ene or cyclopentene) in alkane
solutions at temperatures ranging from 123 to 263 K have led
to the detection of the 16e coordinatively unsaturated species
4
2
roü les of these complexes in the reactions of alkenes that are
catalysed by Group 6 metal carbonyls by focusing on
trans- and cis-[W(CO) (g2-alkene)(solv)] and mer- and fac-
4
bis(alkene) complexes of the type trans-[W(CO) (g2-alkene) ]
[W(CO) (g2-alkene) (solv)] wherein a solvent molecule (solv)
4
2
3
2
and exploiting their photolability with respect to displacement
of CO or alkene ligands and isomerisation. One of the prin-
cipal goals has been to detect, identify and characterise the
occupies the vacant coordination site.20 With an excess
of alkene the 16e tetracarbonyl species react to form cis-
[W(CO) (g2-alkene) ], whereas the 16e tricarbonyl species
4
2
intermediates by which [W(CO) ]-catalysed alkene reactions
form both mer- and fac-[W(CO) (g2-alkene) ]. The identities
6
3
3
evolve.18h20
and structures of the relevant complexes have been assigned
on the basis of the l(CO) bands in their IR spectra.20
Although the synthesis and spectroscopic properties of the
simplest trans bis(alkene) complex, trans-[W(CO) (g2-C H ) ]
Photolysis of [W(CO) ] in the presence of an alkene leads
to the formation of a thermally more stable trans-[W(CO)
6
4
(
g2-alkene) ] complex via a less stable [W(CO) (g2-alkene)]
2
5
4
2 4 2
complex.1,3,4,7h10,17 The cis bis(alkene) complex [W(CO) (g2-
1, have been reported,1,9 neither its structure nor its photo-
chemical behaviour has been established deÐnitively. Here we
report (i) the crystal structure of 1 as determined by X-ray
crystallography and its 1H and 13C NMR properties in
hydrocarbon solution; (ii) the results of matrix-isolation
experiments involving IR detection of the e†ects of selective
irradiation of 1 or its photoproducts; (iii) 1H NMR character-
4
alkene) ], inferior in thermodynamic stability to the trans
2
isomer, has been shown7h11 to function as an intermediate,
and there is general recognition2,21 too that this has a key
roü le to play in catalytic cycles for metathesis, isomerisation
and other reactions of alkenes mediated by [W(CO) ]. Recent
6
studies have demonstrated19 that, under the action of selective
irradiation, reversible photoisomerisation of trans- to cis-
isation of the less stable ethene complexes cis-[W(CO) (g2-
4
[
W(CO) (g2-alkene) ] complexes (alkene \ pent-1-ene or
C H ) ] 2, [W(CO) (g2-C H )] 3, mer-[W(CO) (g2-C H ) ]
4
2
2 4 2
5
3
2 4
2 4 3
3
2 4 3
cyclopentene) can be induced in solid argon matrices at low
temperatures. Several of the more stable trans isomers have
been isolated as solids or liquids that are long-lived at
ambient temperatures;9,17 crystallographic studies reported to
date3,22 reveal structures in which the CxC bonds of the two
alkene ligands are mutually staggered but eclipse the central
4, and fac-[W(CO) (g2-C H ) ] 5, formed by broad-band
UV-visible photolysis of 1 or [W(CO) ] in ethene-saturated
6
hydrocarbon solutions at temperatures in the range 200È293
K; and (iv) exploitation of these 1H NMR spectra for kinetic
measurements of the decay of complexes 2È5 in thermal reac-
tions initiated by dissociation of the ethene ligand.
New J. Chem., 1999, 23, 407È416
407

View Article Online
Table 1 Crystal data and structure reÐnement for trans-[W(CO) (g2-
300 spectrometer operating at 300 and 75.5 MHz, respectively.
4
C H ) ] 1
Spectra of [2H ]toluene solutions were calibrated internally
2
4 2
8
by reference to the methyl resonance of residual [2H ]toluene
Empirical formula
Formula weight
WC H O
7
8
351.99
8 4
occurring at d 2.10 in the 1H and d 20.4 in the 13C NMR
j/Ó
0.710 73
200(2)
Orthorhombic
Aba2
12.458(3)
6.370(2)
12.557(3)
996.5(5)
72 (15 \ h \ 16¡)
4
2.346
11.571
648
3058
spectra, respectively. 1H NMR spectra of [2H ]methyl-
1
4
T /K
cyclohexane solutions were referenced externally to SiMe .
4
Photochemical reactions involving 1 or [W(CO) ] and
6
Crystal system
Space group
ethene occurring in alkane solution at room temperature or in
a/Ó
argon matrices at 14È16 K were investigated by following the
IR spectra of the samples. The products of these reactions in
[2H ]toluene or [2H ]methylcyclohexane solution were
b/Ó
c/Ó
V /Ó3
8
14
Number of reÑections for cell
tracked by 1H NMR measurements at temperatures in the
range 198È350 K. In a typical experiment a solution of 1 (0.15
g, 0.4 mmol) in freshly distilled n-hexane (50 cm3) was saturat-
ed with ethene gas for 15 min and then irradiated through
quartz at about 285 K for 2 h under a continuous ethene
purge. The IR spectrum of the resulting brown solution
exhibited l(CO) bands attributable to 1 and additional fea-
tures at 2050 and 2013 cm~1 indicating the formation of cis-
[W(CO) (g2-C H ) ] 2 (q.v.). After photolysis the solvent was
Z
D /Mg m~3
c
k/mm~1
F(000)
ReÑections collected
Independent reÑections
Goodness-of-Ðt on F2
Conventional R [F [ 4p(F)]
Weighted R (F2 and all data)
884 (R \ 0.0556)
int
1.107
R
\ 0.0221 (635 data)
1
wR \ 0.0506
2
4
2 4 2
removed under vacuum at 265 K, the residual solid dissolved
in [2H ]toluene (0.7 cm3), and the resulting solution trans-
8
Experimental
Materials and synthesis of trans-[W(CO) (g2-C H ) ] 1
ferred to a 5 mm NMR tube. The 1H NMR spectrum of this
sample at 263 K indicated 45% conversion of 1, mainly (45%)
to mer-[W(CO) (g2-C H ) ] 4 (q.v.).
4
2 4 2
3
2 4 3
Synthesis and manipulation of chemicals were carried out
under dry nitrogen using standard Schlenk techniques. Sol-
vents were dried and distilled from CaH under nitrogen prior
The photochemical changes were also followed directly by
1
H
NMR measurements. In
a
typical experiment
a
[2H ]toluene solution ca. 0.02 M in the bis(ethene) complex 1
2
8
to use. Tungsten hexacarbonyl (Aldrich), ethene (Aldrich, 99%
or a [2H ]methylcyclohexane solution ca. 0.1 M in 1 con-
14
grade), [2H ]ethene (C.D.N. Isotopes) and argon (B.O.C.,
tained in a 5 mm NMR tube was cooled to the desired tem-
perature and subjected to broad-band UV-visible irradiation.
The tube was transferred to the precooled sample-holder of
the NMR spectrometer, and the 1H NMR spectrum of the
solution recorded. In some experiments the solvent was satu-
4
“
ResearchÏ grade) were used as supplied. The photolysis
sources were an HBO 200 high-pressure Hg lamp (for synthe-
sis and the solution studies) and a Spectral Energy 1000 W
HgÈXe arc lamp (for the matrix experiments). Selective irra-
diation of matrix samples was carried out with the aid of a
suitable interference Ðlter transmitting light centred at 313 nm
rated with ethene prior to dissolution of 1 or [W(CO) ]. The
6
relative concentrations of the species present in solution were
(
fwhh 16 nm) or 445 nm (fwhh 13 nm).
determined by integrating the 1H NMR signals, the measure-
ments being normalised with respect to the area under the
toluene resonance.
T rans-[W(CO) (g2-C H ) ] 1 was synthesised as follows.
Ethene was bubbled through a solution of [W(CO) ] (0.6 g,
4
2 4 2
6
1.7 mmol) in n-hexane (100 cm3) held at ca. 263 K while being
irradiated with the unÐltered output of the HBO 200 Hg
lamp. The course of the reaction was monitored by IR mea-
surements made on the solution at room temperature, and
photolysis was stopped when the IR band of 1 at 1966 cm~1
reached its maximum intensity (after about 3 h). The volatile
materials were then stripped o† the reaction mixture at ca.
Kinetic studies
The samples destined for kinetic measurements were generally
prepared by the photochemical reaction of ethene with
[W(CO) ] (0.05 M) in [2H ]toluene solution saturated with
6
8
ethene contained in an NMR tube. The sample temperature
was maintained at ca. 200 K prior to NMR measurements.
The rate of decay of a given ethene complex was then deter-
mined by monitoring its 1H NMR signal as a function of time
at a minimum of three di†erent temperatures in the following
ranges: 343È353 K for [W(CO) (C H )] 3, 263È273 K for cis-
273 K and the brown residue was left for 24 h to allow the
monoethene complex [W(CO) (g2-C H )] 3 to decay. The
5
2 4
residue was redissolved in n-hexane (3 cm3) and separation of
1
was achieved by column chromatography on silica using n-
hexane as the eluent. Crystallisation of the product from con-
centrated n-hexane solution at ca. 250 K gave 0.27 g (0.8
mmol) of white crystals of 1 (47% yield based on the amount
5
2 4
[W(CO) (C H ) ] 2, 258È283 K for mer-[W(CO) (C H ) ] 4,
2 4 2 2 4 3
and 228È238 K for fac-[W(CO) (C H ) ] 5.
2 4 3
4
3
3
of [W(CO) ] taken). The product was characterised by its IR
6
and NMR spectra.9,23 IR (hexane): l(CO) 2059vvw, 1997w,
Crystallography
1
966vs, 1936w cm~1. 1H NMR ([2H ]toluene, 293 K): d 2.12
8
(
1
1
s, 2C H , 3J
\ 12.2, 3J
\ 9.9, 2J \ 1.5 Hz).
Details of the crystal data, data collection, structure solution
and reÐnement are given in Table 1. All data were collected
on a StoeŽ Stadi-4 four-circle di†ractometer with graphite-
monochromated Mo-Ka radiation in the uÈ2h mode.
CCDC reference number 440/099. See http://www.rsc.org/
nj/1999/407/ for crystallographic Ðles in .cif format.
2
4
HH(trans)
HH(cis)
HH
3C-M1HN NMR ([2H ]toluene, 293 K): d 198.02 (s, 4CO,
8
J
\ 119.5), 31.20 (s, 2C H , 1J \ 11.3 Hz). 13C NMR
WC
2 4
WC
(
[2H ]toluene, 293 K): d 31.20 (t, 2C H , 1J \ 161.0 Hz).
2 4 CH
8
Spectroscopic and matrix-isolation experiments
IR spectra of solutions were recorded on a Nicolet model 400
FT-IR, spectra of matrices on a Mattson “GalaxyÏ FT-IR
instrument, with a spectral resolution in the second case of
Results and discussion
1
cm~1. Matrices were formed by slow, continuous deposition
Synthesis and spectroscopic characterisation of trans-
[W(CO) (g2-C H ) ] 1
of the vapour of [W(CO) ] or 1 with an excess of argon; the
equipment and operating procedures have been described pre-
6
4
2 4 2
viously.24
Broad-band photolysis of an n-hexane solution of [W(CO) ]
gives in the presence of an excess of ethene at ca. 283 K the
6
1
H and 13C NMR spectra were recorded on a Bruker AMX
408
New J. Chem., 1999, 23, 407È416

View Article Online
bis(ethene) complex trans-[W(CO) (g2-C H ) ]
1 as the
4
2 4 2
major product. The IR spectrum of the solution witnessed ini-
tially the formation of [W(CO) (g2-C H )] 3, with l(CO)
5
2 4
bands at 2086 (0.15), 1973 (0.95) and 1955 (1) cm~1 (relative
intensities being given in parentheses), as illustrated in Fig. 1a.
That photosubstitution of a second CO group then leads to
both trans- and cis-[W(CO) (g2-C H ) ] (1 and 2, respectively)
4
2 4 2
is also signalled by the IR spectrum (Fig. 1b). Thus the forma-
tion of the cis isomer 2 is indicated by the growth of two
l(CO) bands at 2050 and 1913 cm~1 (relative intensities
1
: 1.5); the other two l(CO) bands are obscured by more
intense absorptions associated with 3 and the trans isomer 1.
The cis isomer of [W(CO) (g2-C H ) ] was Ðrst spotted as a
4
2 4 2
thermally labile photoproduct deriving from [W(CO) ] in
6
ethene-doped liquid xenon solution at 195 K, being identiÐed
by its IR spectrum with l(CO) bands at 2050, 1957 and 1910
cm~1.7 The IR spectrum of trans-[W(CO) (g2-C H ) ] 1 is
4
2 4 2
dominated by one intense band at 1966 (1) cm~1 accompanied
by three weaker ones at 2059 (0.003), 1997 (0.11) and 1936
(
0.06) cm~1 (see Fig. 1c). This tallies with previous
reports,7,9,24 the pattern of l(CO) bands mirroring closely that
of other trans-[W(CO) (g2-alkene) ] complexes that have
4
2
been studied previously.1,3,17,19,20 The spectrum can be
interpreted19,24 on the basis of the structure I conforming to
D
symmetry. Compound 1 is long-lived under ambient con-
2d
ditions; it can be stored as an alkane solution in the dark for
months without noticeable change.
Fig. 1 l(CO) region of the IR absorption spectrum of [W(CO) ] H
6
in ethene-saturated n-hexane solution at room temperature: (a) after
broad-band UV-visible photolysis for 90 min at 283 K and (b) after
broad-band UV-visible photolysis for a further 60 min at 283 K,
showing the formation of the ethene carbonyl complexes 1, 2 and 3. (c)
Spectrum of virtually pure trans-[W(CO) (g2-C H ) ] 1.
4
2 4 2
In the 1H NMR spectrum of a [2H ]toluene solution of 1
8
it is necessary to justify the use of the lower symmetry space
group. The pattern of intensities displayed by the reÑections,
allied to symmetry considerations, implied that the W atom
could be placed at (000); the origin was then Ðxed by selecting
a plausible CO unit from a subsequent di†erence synthesis.
The structure was completed by iterative cycles of least-
squares reÐnement and di†erence Fourier synthesis
at 293 K the four ethene hydrogens give rise to a singlet at d
2
.12. With respect to free ethene this resonance shows a
remarkably large shift, *d \ d(free ethene) [ d(coordinated
ethene) \ 3.16. The singlet is accompanied by satellites due to
1
3C-M1HN coupling (1J \ 161 Hz), and each satellite is split
CH
by coupling between the ethene protons with 3J
\ 12.2,
HH(trans)
3J
\ 9.9, and 2J \ 1.5 Hz. Coordination of free ethene
HH(cis)
HH
(SHELXL-9726). In Aba2 the origin is a special position of ..2
thus results in a slight increase in 1J (from 156.425a to 161.0
CH
symmetry, and the molecule is ordered with the two ethene
units staggered with respect to each other: in Cmca the ethene
units are disordered by the inversion centre. H atoms were
located in a di†erence synthesis performed in Aba2 and the
positions reÐned subject to similarity restraints on the CÈH
distances and the geminal and vicinal HÉ É ÉH distances; a
single isotropic displacement parameter was also reÐned for
these atoms. Since the H atom positions were not obvious
from di†erence maps in Cmca, reÐnement in these circum-
stances involved placing the H atoms in ideal positions and
then allowing them to ride on their parent C atoms. ReÐne-
ment in Aba2 converged to R[F [ 4p(F)] \ 0.0221 for 75
parameters, and in Cmca to 0.0267 for 42 parameters. The
additional parameters in the non-centrosymmetric model
comprise the Flack x [0.56(5)], 19 positional parameters
Hz) and an appreciable decrease in the proton coupling con-
stants (3J
\ 19.0, 3J
\ 11.5, and 2J \ 2.3 Hz for
HH(trans)
free ethene25b).
HH(cis)
HH
The 13C resonance moves from d 123.3 in free ethene to d
1.20 in the coordinated ligand in 1. This is the largest low-
3
frequency shift to be reported to date for a trans-[W(CO) (g2-
4
alkene) ] complex.3,9,11,17 The 13C resonance of 1 appears as
2
a triplet with 1J \ 161 Hz and is accompanied by satellites
CH
due to coupling between the ethene carbon and the 183W
nucleus (I \ 1/2, 14.5% natural abundance). The coupling
constant 1J \ 11.3 Hz lends some weight to the description
WC
of the WÈg2-C H unit as a metallocyclopropane. The NMR
2
4
properties indicate overall that each ethene ligand in 1 is
strongly bound, partly through donation of its C -p electrons
2
into a vacant d orbital but mainly through a strong inter-
s
[
W(z), 4 H atoms and one CO group] and 13 displacement
action of the C -p* acceptor orbital with a d occupied orbital
2
p
parameters [12 for the CO groups and one overall U (H)].
of the tungsten. The results are also consistent with the trans
staggered bis(ethene) geometry I inferred from the IR spec-
trum.
iso
The position of the W atom means that it contributes to
only half the data set [k ] l \ 2n for Cmca and h ] k \ 2n for
Aba2]. Although the data exhibit centric intensity statistics
overall, the value of o E2 [ 1o for the weak data originating
exclusively in the light atoms is 0.697 (i.e. strongly acentric). In
addition, R[F [ 0] calculated for all these data is 0.0335 for
the Aba2 reÐnement as against 0.0523 for the Cmca reÐne-
ment. These factors are also reÑected in variance analyses of
F2 against o F2 o / o F2 o which show no abnormal systematic
Crystal structure of trans-[W(CO) (g2-C H ) ] 1
4
2 4 2
Solving the molecular structure of 1 on the basis of X-ray dif-
fraction analysis of a single crystal at 200 K has not proved to
be altogether straightforward. The structure of 1 is reported
here in the space group Aba2. It may also be described,
however, in the related centrosymmetric group Cmca, and so
c
o
c
trends for the non-centrosymmetric space group, but “tailÏ up
New J. Chem., 1999, 23, 407È416
409

View Article Online
to 9.2 for the weakest data in the centrosymmetric group. The
weak data therefore favour the non-centrosymmetric descrip-
tion.
precision of the relevant parameters.
The molecular structure of 1 is illustrated in Fig. 2; bond
distances and angles are listed in Table 2. As
expected,3,9,12,22,27 the CxC bonds of the two g2-
coordinated ethene ligands are staggered with respect to each
While such broad statements cannot be applied to high-
symmetry cases, it remains true that centred orthorhombic
crystal structures are fairly rare and can be mistaken for
twinned monoclinic structures. The structure of 1 may also be
other but eclipse the W(CO) fragment. The preference for this
4
conÐguration is easily understood in terms of optimum
modelled in the P2 /c space group with a cell having the
metal ] p*(alkene) back donation.9,12,27 Because of the
single-faced p-acceptor character of an alkene molecule there
1
dimensions a \ 7.04, b \ 12.45, c \ 6.38 Ó and b \ 116.94¡.
Here we note that the second glide plane conÐrming the
orthorhombic symmetry might easily be missed since it a†ects
a zone of weak reÑections. With the W atom again residing at
will be no competition for metal d electron density between
p
two such molecules if they are in a trans orthogonal orienta-
tion. Measuring 1.413(13) Ó, the CxC bonds of the coordi-
nated ethene molecules are elongated by 0.074 Ó with respect
to the free molecule.28 They are thus roughly halfway between
the bonds in free ethene (r 1.339 Ó)28 and cyclopropane (r
(
000), this structure represents a reduction in symmetry from
2
/m in Cmca to [1 (cf. the reduction to ..2 in Aba2). ReÐne-
ment can be achieved as above to give results broadly similar
to those in the Cmca reÐnement. The apparent disorder in the
ethene units may also be modelled with twinning via a two-
z
z
1.513 Ó).29 Comparisons with other ethene complexes30 show
them to be substantially longer than the corresponding bonds
fold rotation about c
([1 0 [1/0 [1 0/0 0 1). The
monoclinic
twin component reÐnes to a rather low 0.42(1) with a Ðnal
R[F [ 4p(F)] \ 0.0301 and di†erence map extrema of ]1.03
and [1.80 e Ó~3; weak di†erence peaks of 0.68 and 0.58
e Ó~3 are observed in the region of the ethene unit. While these
reÐnement statistics are poorer than those associated with
either of the orthorhombic reÐnements, they are reasonably
acceptable for a compound of this type. The monoclinic model
is, however, chemically di†erent from the orthorhombic ones
because it has mutually eclipsed ethene units. This quite mis-
leading result can again be rejected on the basis of the weak
data for which R[F [ 0] \ 0.060.
Although the crystal structure of 1 is best referred to Aba2,
the structure of the W(CO) unit can be efficiently modelled in
Cmca. It does not seem unreasonable to apply as restraints in
4
Aba2 the constraints on the positional and displacement
parameters of this fragment which are generated when it is
modelled in Cmca. The CÈO and WÈCO bond lengths and the
anisotropic displacement parameters of the centro-
symmetrically related
C and O atoms were therefore
restrained to be similar. Although this procedure barely
a†ected the agreement indices or convergence, it improved the
Fig. 2 Structure of trans-[W(CO) (g2-C H ) ] as determined by
4
2 4 2
X-ray di†raction of a single crystal at 200 K.
Table 2 Bond lengths (Ó) and angles (¡) for crystalline trans-[W(CO) (g2-C H ) ] 1a
4 2
4
2
W(1)ÈC(2)
W(1)ÈC(2i)
W(1)ÈC(1)
W(1)ÈC(1i)
W(1)ÈC(4)
W(1)ÈC(4i)
W(1)ÈC(3)
W(1)ÈC(3i)
2.033(10)
2.033(10)
2.045(9)
2.045(9)
2.299(9)
2.299(9)
2.315(9)
2.315(9)
C(1)ÈO(1)
C(2)ÈO(2)
C(3)ÈC(4)
C(3)ÈH(31)
C(3)ÈH(32)
C(4)ÈH(41)
C(4)ÈH(42)
1.151(11)
1.142(10)
1.413(13)
0.98(4)
0.98(4)
0.98(4)
0.98(4)
C(2)ÈW(1)ÈC(2i)
C(2)ÈW(1)ÈC(1)
C(2i)ÈW(1)ÈC(1)
C(2)ÈW(1)ÈC(1i)
C(2i)ÈW(1)ÈC(1i)
C(1)ÈW(1)ÈC(1i)
C(2)ÈW(1)ÈC(4)
C(2i)ÈW(1)ÈC(4)
C(1)ÈW(1)ÈC(4)
C(1i)ÈW(1)ÈC(4)
C(2)ÈW(1)ÈC(4i)
C(2i)ÈW(1)ÈC(4i)
C(1)ÈW(1)ÈC(4i)
C(1i)ÈW(1)ÈC(4i)
C(4)ÈW(1)ÈC(4i)
C(2)ÈW(1)ÈC(3)
C(2i)ÈW(1)ÈC(3)
C(1)ÈW(1)ÈC(3)
C(1i)ÈW(1)ÈC(3)
C(4)ÈW(1)ÈC(3)
C(4i)ÈW(1)ÈC(3)
92.3(9)
171.8(7)
90.9(3)
90.9(3)
171.8(7)
87.0(9)
75.4(5)
87.1(5)
112.3(5)
86.3(5)
87.1(5)
75.4(5)
86.3(5)
112.3(5)
154.9(5)
111.1(4)
86.3(4)
76.7(5)
85.5(5)
35.7(3)
154.8(3)
C(2)ÈW(1)ÈC(3i)
C(2i)ÈW(1)ÈC(3i)
C(1)ÈW(1)ÈC(3i)
C(1i)ÈW(1)ÈC(3i)
C(4)ÈW(1)ÈC(3i)
C(4i)ÈW(1)ÈC(3i)
C(3)ÈW(1)ÈC(3i)
O(1)ÈC(1)ÈW(1)
O(2)ÈC(2)ÈW(1)
C(4)ÈC(3)ÈW(1)
C(4)ÈC(3)ÈH(31)
W(1)ÈC(3)ÈH(31)
C(4)ÈC(3)ÈH(32)
W(1)ÈC(3)ÈH(32)
H(31)ÈC(3)ÈH(32)
C(3)ÈC(4)ÈW(1)
C(3)ÈC(4)ÈH(41)
W(1)ÈC(4)ÈH(41)
C(3)ÈC(4)ÈH(42)
W(1)ÈC(4)ÈH(42)
H(41)ÈC(4)ÈH(42)
86.3(4)
111.1(4)
85.5(5)
76.7(5)
154.8(3)
35.7(3)
155.4(5)
175.1(13)
176.2(14)
71.5(5)
121(4)
115(5)
131(4)
117(6)
99(5)
72.8(5)
134(4)
110(5)
124(4)
111(5)
99(5)
a Symmetry transformations used to generate equivalent atoms: i 1 [x, [y, z.
410
New J. Chem., 1999, 23, 407È416

View Article Online
in K[PtCl (g2-C H )] É H O [1.375(4) Ó]31a and [Pt(g2-
2 4
C H ) (g2-C F )] [1.365(8) Ó],31b comparable with those in
4 2 2 4
Pt(g2-C H ) ] [1.402(7) Ó]31b and the tungsten(IV) com-
4 3
pound [W(O)Cl (g2-C H )(PMePh ) ] [1.404(17) Ó],31c but
2 4 2 2
shorter than those in mer-[W(CO) (g2-C H )(g4-nor)]
2 4
nor \ norbornadiene) [1.48(1) Ó]11 and [Ta(g5-C Me )(g2-
[W(CO) ]Èalkene systems.1,9,19,20 Switching to radiation with
3
2
6
j \ ca. 445 nm then reverses the substitution process with the
2
[
regeneration of [W(CO) ] and the pentacarbonyl species (see
2
6
Table 3). The rigid matrix permits the trapping of 16e and 14e
2
intermediates like [W(CO) (Ar) ] (n \ 4 or 5) but su†ers
3
n
6~n
(
from the disadvantage of restricting severely the formation of
products incorporating two or more ethene ligands.
5
5
C H )(CHCMe )(PMe )] [1.477(4) Ó].31d As in other ethene
2
4
3
3
complexes, the hydrogen atoms of the C H ligands are bent
A clearer picture of the photochemistry of the bis(ethene)
complexes [W(CO) (C H ) ] is gained from experiments with
2
4
away from the metal giving an angle (a) of about 37¡ between
the normals to the two methylene planes. There are various
factors (the nature of the metal centre, the overall stereoche-
mistry at this centre, and the e†ects of other ligands) which
inhibit any clearcut correlation between the dimensions of the
metalÈC H unit and the nature and strength of the bonding.
4
2 4 2
trans-[W(CO) (g2-C H ) ] isolated in an undoped argon
4
2 4 2
matrix. As illustrated in Fig. 3 and detailed in Table 4, irradia-
tion of the matrix with light Ðrst at j \ ca. 313 nm and then
at j \ ca. 445 nm follows exactly the pattern of behaviour
noted previously in similar studies of the bis(cyclopentene)
and bis(pent-1-ene) complexes.19 Thus, photolysis at j \ ca.
313 nm results in isomerisation of trans- to cis-[W(CO) (g2-
C H ) ] as the primary photoprocess, whereas this isomer-
isation is reversed by visible light centred near j \ 445 nm
eqn. (1)]. There are also signs of secondary products, notably
2
4
However, the results of the present study are in keeping with
the NMR properties of trans-[W(CO) (g2-C H ) ] (q.v.), while
4
2 4 2
4
coming perhaps less close to a metallocyclopropane formula-
tion than these properties might suggest.
2
4 2
[
Photochemistry of [W(CO) ] + C H and trans-[W(CO) (g2-
6
2 4
4
C H ) ] : matrix-isolation studies
2
4 2
Irradiation of [W(CO) ] isolated in an ethene-doped (2%)
6
argon matrix at 16 K with UV light at j \ ca. 313 nm results
in the decay of the absorptions associated with the hexa-
carbonyl and the appearance and growth of new absorptions
in the l(CO) region of the IR spectrum. Comparison with
earlier studies19,32 enables these new features to be identiÐed
with the formation of free CO and the tungsten carbonyls
[W(CO) (g2-C H )(Ar)] and [W(CO) (Ar) ],32 implying that
4
2 4
4
2
isomerisation is initiated not intramolecularly but by photo-
[
W(CO) (Ar)], [W(CO) (g2-C H )], and cis- and trans-
2 4
W(CO) (g2-C H ) ]. Such experiments are hampered,
dissociation of a WÈC H bond. Once again, however, the
5
5
2 4
[
chemical changes open to the molecules are restricted by the
4
2 4 2
however, by the appearance of bands at ca. 2040 and 1894
cm~1 exhibited by the matrix initially and which grew on
photolysis, thereby obscuring important regions of the spec-
trum. These errant bands are associated with loosely bound
rigidity of the matrix support and the inability of a product
molecule like C H to escape from the matrix cage wherein it
2
4
is generated.
[
C H ] dimers present in the matrix.33
4 2
To overcome the problems of ethene aggregates, similar
2
Solution photochemistry of [W(CO) ] + ethene: 1H NMR
6
studies
experiments have been carried out with [W(CO) ] isolated in
6
a [2H ]ethene-doped argon matrix. Irradiation at j \ ca. 313
4
The progress of the changes occurring on photolysis of an
nm can then be seen initially to give rise to free CO and the
ethene-saturated [2H ]toluene solution containing [W(CO) ]
8
6
pentacarbonyl derivatives [W(CO) (Ar)] and [W(CO) (g2-
C D )] (see Table 3). Continued irradiation results in the for-
mation of the tetracarbonyl derivatives cis-[W(CO) L ] (L \
C D
at about 200 K has also been tracked by 1H NMR measure-
ments. Hence it has been possible to detect 18e ethene car-
bonyl complexes that are relatively short-lived at ambient
temperatures. Fig. 4 illustrates the 1H signals which appear
after broad-band UV-visible photolysis of such a solution at
213 K. The only new signal with signiÐcant intensity to be
observed after brief photolysis occurring at d 2.40 must be
attributed to the mono-ethene complex [W(CO) (g2-C H )]
5
5
2
4
4
2
or
Ar),
cis-[W(CO) (C D )(Ar)],
trans-
2
4
4
2 4
[W(CO) (C D ) ] and trans-[W(CO) (C D )(Ar)] at the
4
2 4 2
4 2 4
expense of the pentacarbonyl intermediates. The relevant mol-
ecules can be identiÐed by their IR spectra and by drawing on
the results of previous photochemical studies of
5
2 4
Table 3 l(CO) region of IR spectrum of [W(CO) ] trapped in a C D -doped (2%) argon matrix at ca. 16 K showing the e†ects of sequential
6
2 4
irradiation with UV and visible light
Sequential photolysis at ja/nm
3
13
313
(3360 s)
313
(4260 s)
445
(600 s)
445
(1200 s)
313
(300 s)
l
8
a/cm~1
Depositiona
(1560 s)
Assignmentb
2
2
2
2
2
1
1
1
1
1
1
1
1
1
137.4
089.1
056.8
051.5
034.2
991.7
986.4
976.3
962.8
961.3
955.6
932(br)
909.8
893.9
[
[
[
[
[
m
vvw
vvw
[
vvw
[
B
C
C
B
C
B
C
B
B
B
B
[
B
B
C
C
\
C
B
C
C
C
C
C
c
m
C
B
B
B
\
C
C
B
C
B
C
CO
\
vvw
C
[W(CO) (L)], a
5
1
[
\
\
C
cis-[W(CO) (L) ], a
4
2
1
cis-[W(CO) (L)(Ar)], a
4
1
vvw
B
mer-[W(CO) (L) ]
n
p
3
3
B
o
[W(CO) ], t
vs
B
B
C
B
B
C
B
B
C
C
C
6
1u
[
[
[
[
[
[
[
[
sh
C
C
[W(CO) (L)], e
[W(CO) (Ar)], e
5
s
c
5
[
mw
w
[
sh
C
C
C
[
\
[
trans-[W(CO) (L) ], e
4
[W(CO) (L)], a
5
2
1
[W(CO) (Ar)], a
5
1
[
w
w
cis-[W(CO) (L) ], b
cis-[W(CO) (Ar) ], b
4
2
2
[
4
2
2
a C increases; B decreases; \ unchanged; [ absent; s strong; m medium; w weak; v very; sh shoulder; br broad. b L \ C D . c Obscured.
2
4
New J. Chem., 1999, 23, 407È416
411

View Article Online
Fig. 4 1H NMR spectra illustrating the thermal decay of the tung-
sten ethene carbonyl complexes 2, 3, 4 and 5 in [2H ]toluene solution
8
as the temperature is increased from 213 to 293 K in 10 K intervals.
The compounds were generated by broad-band UV-visible photolysis
of [W(CO) ] in an ethene-saturated solution at ca. 200 K. S denotes a
6
solvent resonance.
the e†ects of photolysis time allied to the conclusions drawn
from IR studies regarding the evolution and distribution of a
given product; (iv) analogies with the NMR properties of
related complexes; and (v) the thermal decay properties of the
product. In connection with (iv) we note that the signal due to
the two mutually trans ethene ligands in mer-[W(CO) (g2-
3
C H ) ] 4 occurs in almost the same position as the ethene
2
4 3
resonance of mer-[W(CO) (g2-C H )(g4-nor)] (d 1.55),11 and
Fig. 3 l(CO) region of the IR absorption spectrum of trans-
3
2 4
that the ethene resonance in [W(CO) (g2-C H )] 3 is close to
2 4
that in [Cr(CO) (g2-C H )] (d 2.67 in [2H ]benzene
2 4
[
W(CO) (g2-C H ) ] 1, isolated in an argon matrix at ca. 14 K
4
2
4 2
5
showing the e†ects of sequential UV and visible photolysis: (a) prior
to photolysis; (b) after photolysis at j \ ca. 313 nm for 60 min,
showing isomerisation to cis-[W(CO) (g2-C H ) ] 2; and (c) after
5
6
solution).15
Raising the temperature of the photolysed solution causes
several of the 1H signals to decay, and at room temperature
the spectrum consists only of the features at d 2.12 and 2.84
originating in the complexes 1 and 3, respectively (Fig. 4).
4
2 4 2
photolysis at j \ ca. 445 nm for 60 min, showing regeneration of 1.
3
. Prolonged photolysis then gives rise to additional 1H reso-
nances originating in bis(ethene) and tris(ethene) derivatives.
With [2H ]toluene as the solvent the chemical shifts of most
8
Those at d 2.02 and 2.01 can be associated with cis- and trans-
of the 1H signals due to the ethene complexes move to signiÐ-
[
W(CO) (g2-C H ) ], i.e. 2 and 1 respectively, whereas two
cantly higher frequency as the temperature increases in the
range 198È293 K. Only the signal due to 1 and the low-
frequency signal of 4 su†er little change of chemical shift (Fig.
4); most conspicuous are the high-frequency shifts displayed
4
2 4 2
resonances having a common origin at d 2.54 and 1.52 and
with relative intensities of 1:2 clearly belong to mer-[W(CO)
3
(g2-C H ) ] 4. The only sign of fac-[W(CO) (g2-C H ) ] 5,
2
4 3
3
2 4 3
is a resonance of very low intensity at d 2.45 (see Fig. 4).
Various criteria have been applied to identify positively the
carriers of the di†erent signals: (i) the known NMR properties
of the stable complex 1; (ii) the distinctive spectrum of 4; (iii)
by the protons of the compounds 2 and 3. With
[2H ]methylcyclohexane as the solvent, however, the 1H
1
4
shifts vary but little with temperature. Table 5 lists the 1H
chemical shifts as functions of both solvent and temperature.
Table 4 l(CO) region of IR spectrum of trans-[W(CO) (g2-C H ) ] 1 trapped in an argon matrix at ca. 14 K showing the e†ects of sequential
4 2
4
2
irradiation with UV and visible light
Sequential photolysis at ja/nm
3
13
313
(3900 s)
445
(1800 s)
l
8
/cm~1
Depositiona
(900 s)
Assignmentb
2
2
1
1
1
1
1
1
1
1
1
1
057.3
053.9
999.0
972.9
967.6
957.5
954.1
948.4
936.8
918.0
897.2
894.4
[
[
w
[
vs
[
[
[
w
w
vw
B
C
C
B
B
B
C
C
d
B
C
C
C
B
C
C
C
C
B
B
m
C
C
B
B
cis-[W(CO) (L) ], a
4
2
1
trans-[W(CO) (L)(Ar)], a
4
1
trans-[W(CO) (L) ], ac
4
[W(CO) (L)], e
5
2
1
m
B
trans-[W(CO) (L) ], e
4 2
cis-[W(CO) (L) ], b
s
s
d
\
w
m
m
4 2
1
1
cis-[W(CO) (L) ], a
4
2
[W(CO) (L)], a
5
1
trans-[W(CO) (L) ], ec
4
2
[
[
[
trans-[W(CO) (L)(Ar)], e
4
cis-[W(CO) (L) ], b
4
2
2
cis-[W(CO) (Ar) ], b
4
2
2
a C increases; B decreases; \ unchanged; [ absent; s strong; m medium; w weak; v very. b L \ C H . c trans-[W(12CO) (13CO)(C H ) ].
4 2
2
4
3
2
d Obscured.
412
New J. Chem., 1999, 23, 407È416

View Article Online
Table 5 1H chemical shifts for di†erent tungsten ethene carbonyl complexes 1È5 as a function of solvent and temperature
C D , d C D , d
7
14
7 8
d \ xT ] ya
Complex
W(CO) (g2-C H )] 3
293 K
3.50
213 K
3.49
293 K
2.84
213 K
2.40
104x
47b
y
[
1.47b
0.66c
2.01
1.43
1.83
1.04
1.71f
5
2 4
8
1c
mer-[W(CO) (g2-C H ) ] 4
3.13
3.11
1.67
3.05
2.83
2.33
2.73
1.56
2.68e
2.39
2.12
2.54
1.52
2.45
2.02
2.01
25
4
29
46
14f
3
2
4 3
1.70
fac-[W(CO) (g2-C H ) ] 5
4 3
cis-[W(CO) (g2-C H ) ] 2
4 2
trans-[W(CO) (g2-C H ) ] 1
4 2
3.09d
2.91
2.35
3
2
4
2
4
2
a Equation of the linear plot of d vs. T in the temperature range T \ 198È293 K unless stated otherwise. b Applies to the temperature range
T \ 238È293 K. c Applies to the temperature range T \ 198È233 K. d Value calculated from the equation d \ 0.0005T ] 2.95. e Value calculated
from the equation d \ 0.0029T ] 1.83. f Applies to the temperature range T \ 203È353 K.
[
2H ]Toluene solutions give shifts which vary linearly with
8
temperature in the range 198È293 K, except in the case of 3
which is characterised by two distinct temperature regimes of
linear dependence (198È233 and 238È293 K, see Table 5).
These results point to solvent interactions which are more
speciÐc for the unsaturated toluene than for the saturated
methylcyclohexane molecules; possibly etheneÈtoluene
exchange is a signiÐcant factor for the more labile WÈC H
correlates with the accretion of 3 [as in eqn. (3)]. These con-
clusions are supported by the rate constants (q.v.) for the dis-
appearance of 4 and 2 which match those for the recovery of 1
2
4
moieties.
For the unstable ethene complexes 2È5, the change in 1H
chemical shift with respect to free ethene, *d, for the ethene
ligand trans to a CO group increases from 2.4 to 2.9 in the
order 3 \ 4 \ 5 \ 2. For the mutually trans ethene ligands of
1
and 4, by contrast, *d is substantially larger, viz, 3.16 and
3.72, respectively. This di†erence reÑects on the one hand the
competition that exists with CO, a better p-acceptor than
ethene, for the d orbitals of tungsten, thereby weakening the
p
and 3, respectively, implying simple interconversion reactions
linking these two pairs of species. The decay of 5 at tem-
peratures below 253 K appears to proÐt 2, indicating the
interconversion (4). Hence the complexes 2, 4 and 5 all decay
trans-WÈC H interaction, and on the other the lack of such
2
4
competition between two trans, mutually staggered ethene
ligands.
Least stable of the ethene complexes is fac-[W(CO) (g2-
3
C H ) ] 5, which can be detected only at low temperatures, so
2
4 3
that the chemical shift of d 2.68 at 293 K given in Table 5 has
to be extrapolated from the temperature-dependence of the
measured shift at temperatures \ 273 K. IR studies of alkane
solutions have demonstrated that cis-[W(CO) (g2-C H ) ]
4
3 6 2
survives at temperatures as high as 263 K;20 on the evidence
of the present studies the corresponding bis(ethene) complex,
2
, survives long enough in toluene or methylcyclohexane solu-
tion to be detected even at 293 K. Over time the resonances of
, 4 and 5 decrease in intensity while those of 1 and 3 are
thermally via reactions in which the CO present in the pho-
tolysed reaction mixture displaces a coordinated ethene
ligand. The 1H signal due to the monoethene complex 3 is
observed to decay at a measurable rate only at temperatures
in the range 343È353 K, again with the release of free ethene
2
observed to grow (Fig. 4 and 5). If the temperature of the
solution is raised above 228 K, the decay of 4 can be wit-
nessed. No additional intermediate being detectable, the con-
sumption of 4 must be supposed to give rise only to
compound 1 via the substitution reaction (2) (see Fig. 5). Simi-
larly we Ðnd that the decay of 2 at temperatures above 253 K
and, presumably, the formation of [W(CO) ]:
6
Hence the results presented here leave little room for doubt-
ing the identities of the labile ethene carbonyl derivatives
responsible for the di†erent 1H resonances.
Solution photochemistry of trans-[W(CO) (g2-C H ) ] 1: 1H
4
2 4 2
NMR studies
IR measurements made at room temperature indicate that
broad-band UV photolysis of 1 in ethene-saturated hexane
solution at ca. 265 K yields initially the cis isomer 2 as the
Fig. 5 Thermal reactions of ethene carbonyl complexes of tungsten
in [2H ]toluene solution as the temperature is raised from 213 to 293
8
K.
New J. Chem., 1999, 23, 407È416
413

View Article Online
(up to 93% in a typical experiment) when photolysis is carried
out in the presence of an excess of ethene (see Scheme 1).
Thus, an excess of ethene suppresses the isomerisation of 1 to
2
, tending instead to favour the regeneration of 1. At the same
time the photochemical conversion of 1 to other products
decreases (typically from 40 to 26%) when the solution con-
tains an excess of ethene. These results therefore endorse the
inference drawn from earlier IR measurements19,20 that trans-
to-cis isomerisation occurs through photolytic alkene loss, for-
mation and pseudo-rotation of the coordinatively unsaturated
species [W(CO) (g2-alkene)], and subsequent re-coordination
4
of the alkene molecule to give the relatively stable cis-
[
W(CO) (g2-alkene) ]. Of thermal interconversion of the two
4
2
isomers, however, we Ðnd no evidence.
Broad-band photolysis of 1 also produces small amounts of
the monoethene complex 3 (Scheme 1). Hence ethene-loss
from [W(CO) (g2-C H ) ] competes with CO-loss and
4
2 4 2
whether the unsaturated species [W(CO) (g2-C H )] takes up
4
2 4
CO or C H depends upon the conditions of the experiment
and the composition of the solvation shell enclosing the reac-
tion intermediate.
2
4
Scheme 1 The yields of ethene carbonyl complexes formed after
broad-band UV-visible photolysis of trans-[W(CO) (g2-C H ) ] 1 at
4
2 4 2
ca. 200 K for 2 h. The proportions determined by 1H NMR measure-
ments are reported with respect to the total yield of products at 213
K; Ðgures in parentheses relate to the e†ects of photolysis in the pres-
ence of an excess of ethene.
Kinetics of the disappearance of the ethene carbonyl complexes
2, 3, 4 and 5
sole detectable product, but that prolonged photolysis causes
both the bis(ethene) complexes to give place to the tris(ethene)
isomers 4 and 5. These products could be detected by their 1H
NMR spectra by evaporating the hexane at 265 K and dis-
To observe the labile ethene carbonyl complexes, we Ðrst
exposed a [2H ]toluene solution of [W(CO) ] with an excess
8
6
of ethene in an NMR tube at ca. 200 K to broad-band UV-
visible radiation for ca. 2 h. As noted previously, all four
ethene complexes 2, 3, 4 and 5 decay with the dissociation of
free ethene at temperatures [200 K. The rate of disap-
pearance of each of the complexes has been monitored by ref-
erence to the integrated intensities of the relevant 1H signals;
this has been carried out in the presence of 1 to 5 equivalents
of ethene over times spanning at least three half-lives under
pseudo-Ðrst-order conditions. The measurements have been
made for the decay of each of the four complexes 2È5 at a
minimum of three di†erent temperatures. The rate constants k
estimated in this way give linear ln k vs. 1/T plots, tending to
conÐrm the Ðrst-order dependence of the rate on the concen-
tration of the ethene complex. Various experiments were
carried out in an e†ort to ascertain the e†ects of adding CO to
the photolysed reaction mixture (e.g. by bubbling the gas
through the sample in the NMR tube). There was some accel-
eration of the decay of the photo-products, amounting in the
case of the complex 3 to a roughly two-fold increase in the
rate, but it proved impossible to secure more quantitative
results. In the circumstances, the pseudo-Ðrst-order rate con-
stants reported here relate to conditions which were stan-
dardised as far as possible giving reproducible results, with the
CO deriving exclusively from the initial photolysis stages; they
solving the residue in [2H ]toluene. On the evidence of the
8
NMR spectrum of the resulting solution, the reaction mixture
contained typically 1 (55%), 4 (40%), 5 (\1%), and also 3
(
2%). The high concentration of 4 thus produced was made
possible by the removal of CO from the reaction mixture
during photolysis, thereby inhibiting the regeneration of the
more stable bis(ethene) complex 1. These Ðndings tend to
conÐrm the results of earlier studies to the e†ect that UV pho-
tolysis of 1 favours cleavage of WÈCO rather than WÈC H
2
4
bonds, yielding mainly mer-[W(CO) (g2-C H ) ] 4.
3
2 4 3
Broad-band UV-visible photolysis of 1 in [2H ]toluene
solution contained in an NMR tube at about 200 K enabled
8
us to observe the formation of all the products 2È5 in the
proportions shown in Scheme 1. The major product is now
the tris(ethene) complex 4 (57%), a result consistent with a
pathway involving initial CO loss to form coordinatively
unsaturated [W(CO) (g2-C H ) ] followed by trapping of free
3
2 4 2
C H . Such a mechanism will obviously be inhibited by the
2
4
presence in excess of the leaving ligand. Hence it is note-
worthy that the yields of the CO-rich complexes 2 and 3 are
higher when photolysis is carried out in the absence of addi-
tional ethene. By contrast, the yields of 4 increase dramatically
Table 6 Activation energies, E , and pseudo-Ðrst-order rate constants, k, for the thermal decay of the tungsten ethene carbonyl complexes 2È5
a
by reaction with CO in [2H ]toluene solution
8
Complex
W(CO) (g2-C H )] 3
E /kJ mol~1
T /K
10~3q/s
104k/s~1
a
[
198
121
65
343
11.2
3.5
1.6
11.2
2.2
1.5
7.5
6.1
1.6
0.6
2.5
1.4
1.0
0.6
1.9
4.3
0.6
3.0
4.5
0.9
1.1
4.1
11.6
2.7
4.9
6.5
5
2 4
3
3
48
53
cis-[W(CO) (g2-C H ) ] 2
4 2
263
4
2
2
2
68
73
mer-[W(CO) (g2-C H ) ] 4
4 3
258
3
2
2
2
2
68
70
83
fac-[W(CO) (g2-C H ) ] 5
4 3
40
228
3
2
2
2
33
38
414
New J. Chem., 1999, 23, 407È416

View Article Online
must therefore be viewed as order-of-magnitude rather than
deÐnitive estimates, but giving signiÐcant comparative mea-
sures, as well as rough guides to the relevant activation bar-
riers. Table 6 gives a summary of the kinetic results, including
the rate constants measured at di†erent temperatures and
which activation energies E have been deduced for the decay
a
reactions. Hence E has been found to decrease in the order
a
3 [ 2 [ 4 [ 5. fac-[W(CO) (g2-C H ) ] is the least stable of
3
2 4 3
the ethene complexes, giving place to cis-[W(CO) (g2-C H ) ]
4
2 4 2
in a reaction with E \ ca. 40 kJ mol~1. By contrast, the con-
a
activation energies E deduced from the appropriate Arrhe-
version of [W(CO) (g2-C H )] to [W(CO) )] is opposed by
a
5
2 4
6
nius plots. Hence it emerges that the exchange reactions are
E \ ca. 198 kJ mol~1. These values compare sensibly with
a
opposed by E values which decrease in the order 3 [reaction
analogous parameters reported for other metalÈethene com-
a
(
5)] [ 2 [reaction (3)] [ 4 [reaction (2)] [ 5 [reaction (4)].
plexes and may be expected to represent lower limits to the
All the signs are that the substitution reactions are activated
dissociation energies of the appropriate WÈC H bonds. Our
2
4
by dissociation of a tungstenÈethene bond,19,20 so that the E
experiments a†ord no evidence to suggest that the intercon-
version of the trans and cis bis(ethene) complexes 1 and 2 can
be realised by thermal means.
a
values represent lower limits for the dissociation energy of this
bond. It is signiÐcant then that the unimolecular decay con-
stant we have extracted from the NMR measurements for cis-
[
W(CO) (g2-C H ) ] 2 is comparable with the corresponding
2 4 2
value reported for cis-[Cr(CO) (g2-C H ) ] in the gas
2 4 2
phase.10 At 198 kJ mol~1, the activation energy for reaction
4
Acknowledgements
We acknowledge with gratitude the award of a research grant
4
(
5) is substantially larger than the values variously estimated
(to T.S.-B. and A.J.D.) by the British Council, and support
for the corresponding reaction of [Cr(CO) (g2-C H )] in
5
2 4
from the E.P.S.R.C. for the purchase of equipment at both
Oxford and Edinburgh and the funding of an Advanced Fel-
lowship (to T.M.G.) and a research studentship (to L.J.M.).
S.P. would like to thank Dr. R. O. Gould for helpful dis-
cussions. We thank also Mr. E. J. J. Riddle for his assistance
with the matrix-isolation experiments.
liquid xenon solution (*Ht \ 60 kJ mol~1)7 or in the gas
phase (E \ 105 kJ mol~1).34 It is also larger than the value of
a
9
7
kJ mol~1 opposing ethene loss from [(g5-
C H )Nb(CO) (g2-C H )] in supercritical ethene solution.35
5
5
3
2 4
The pattern is entirely consistent with the importance of
metalÈethene bond-breaking in the rate-determining
step7,19,20,34,35 since third-row transition metals form appre-
ciably stronger bonds to ethene than do Ðrst- or second-row
transition metals. However, meaningful estimates of *St
associated with the present CO-for-C H substitution reac-
Notes and references
1
2
3
4
5
I. W. Stolz, G. R. Dobson and R. K. Sheline, Inorg. Chem., 1963, 2,
2
4
1
264.
tions must await further experiments in which the concentra-
tion of CO is a known and controllable quantity.
M. Wrighton, G. S. Hammond and H. B. Gray, J. Organomet.
Chem., 1974, 70, 283.
F.-W. Grevels, M. Lindemann, R. Benn, R. Goddard and C.
KruŽ ger, Z. Naturforsch., T eil B, 1980, 35, 1298.
Conclusions
F.-W. Grevels and V. Skibbe, J. Chem. Soc., Chem. Commun., 1984,
The relatively stable bis(ethene) complex trans-[W(CO) (g2-
C H ) ] 1 has been characterised not only by its spectro-
681.
4
K. Angermund, F.-W. Grevels, C. Kru
Chem., Int. Ed. Engl., 1984, 23, 904.
K. R. Pope and M. S. Wrighton, Inorg. Chem., 1985, 24, 2792.
M. F. Gregory, S. A. Jackson, M. Poliako† and J. J. Turner, J.
Chem. Soc., Chem. Commun., 1986, 1175.
Ž ger and V. Skibbe, Angew.
2
4 2
scopic properties but also by its crystal and molecular struc-
ture, as determined by X-ray crystallography. The results
conÐrm that the CxC bonds of the two ethene ligands are
6
7
mutually staggered while eclipsing the central W(CO) frame-
8 S. A. Jackson, R. K. Upmacis, M. Poliako†, J. J. Turner, J. K.
4
work, and that the WÈC H interaction is a comparatively
strong one, while still falling short of the metallocyclopropane
limit. Reversible isomerisation of trans- to cis-[W(CO) (g2-
C H ) ] 2 has been shown by IR measurements to be the
primary change occurring on selective UV or visible photoly-
sis of solid argon matrices at 14È16 K. In addition, 1H NMR
measurements have been applied for the Ðrst time to the iden-
Burdett and F.-W. Grevels, J. Chem. Soc., Chem. Commun., 1987,
2
4
6
78.
F.-W. Grevels, J. Jacke and S. O
7536.
0 B. H. Weiller and E. R. Grant, J. Am. Chem. Soc., 1987, 109, 1252.
11 F.-W. Grevels, J. Jacke, P. Betz, C. Kruger and Y.-H. Tsay,
Organometallics, 1989, 8, 293.
9
 zkar, J. Am. Chem. Soc., 1987, 109,
4
1
2
4 2
Ž
12 C. Daniel and A. Veillard, Inorg. Chem., 1989, 28, 1170.
1
3 S. A. Jackson, P. M. Hodges, M. Poliako†, J. J. Turner and F.-W.
Grevels, J. Am. Chem. Soc., 1990, 112, 1221.
tiÐcation of the labile ethene complexes cis-[W(CO) (g2-
4
C H ) ] 2, [W(CO) (g2-C H )] 3, mer-[W(CO) (g2-C H ) ]
2
4 2
5
3
2 4
3
2 4 3
14 P. M. Hodges, S. A. Jackson, J. Jacke, M. Poliako†, J. J. Turner
and F.-W. Grevels, J. Am. Chem. Soc., 1990, 112, 1234.
15 J. A. Banister, S. M. Howdle and M. Poliako†, J. Chem. Soc.,
Chem. Commun., 1993, 1814.
4
, and fac-[W(CO) (g2-C H ) ] 5, formed by broad-band
2 4 3
photolysis of ethene-saturated hydrocarbon solutions of 1 or
[W(CO) ] in the temperature range 198È293 K. Only by pho-
6
tolysis at low temperatures has it been possible to generate
these species at concentrations high enough to permit their
detection by 1H NMR measurements. The results of these
experiments conÐrm that tungstenÈethene photodissociation
is the principal reaction pathway in the photolysis of 1, initi-
ating the photoisomerisation of the trans to the cis isomer 2
or, in the presence of free CO, giving access to the mono-
ethene complex [W(CO) (g2-C H )].
16 H. Takeda, M. Jyo-o, Y. Ishikawa and S. Arai, J. Phys. Chem.,
1
995, 99, 4558.
7 M. Jaroszewski, T. Szyman
Zio\kowski, J. Organomet. Chem., 1996, 509, 19.
8 T. Szymanska-Buzar, M. Jaroszewski, M. Wilgocki and J. J.
Zio\kowski, J. Mol. Catal. A, 1996, 112, 203.
19 T. Szymanska-Buzar, M. Jaroszewski, A. J. Downs, T. M. Greene
and L. J. Morris, J. Organomet. Chem., 1997, 531, 207.
0 T. Szymanska-Buzar, K. Kern and D. J. Stufkens, New J. Chem.,
1998, 22, 1539.
1 T. Szyman
1
1
 ska-Buzar, M. Wilgocki and J. J.




2

5
2 4
Of particular interest are the Ðrst kinetic studies of the
2
2
 ska-Buzar, J. Mol. Catal., 1991, 68, 177.
thermal decay of the ethene complexes 2È5 made possible by
2 J. M. Dalla Riva Toma, P. H. Toma, P. E. Fanwick, D. E. Bergs-
1
H NMR measurements on hydrocarbon solutions at tem-
trom and S. R. Byrn, J. Crystallogr. Spectrosc. Res., 1993, 23, 41.
peratures ranging from 198 to 353 K. The decay proceeds in
each case by the substitution of CO (formed during
photolysis) for ethene, complex 4 transforming to 1, 2 to 3, 5
23 G. Davidson and C. L. Davies, Inorg. Chim. Acta, 1989, 165, 231.
2
4 T. Szyma n ska-Buzar, A. J. Downs, T. M. Greene and A. S. Mar-
shall, J. Organomet. Chem., 1995, 495, 149, 163.
2
5 (a) G. C. Levy, R. L. Lichter and G. L. Nelson, Carbon-13 Nuclear
Magnetic Resonance Spectroscopy, Wiley, New York, 2nd edn.,
to 2, and 3 to [W(CO) ] [eqn. (2), (3), (4) and (5), respectively].
6
By reference to the integrated intensities of the relevant 1H
1
980, p. 89; (b) F. A. Bovey, Nuclear Magnetic Resonance Spectros-
copy, Academic Press, 2nd edn., 1988, p. 612.
26 G. M. Sheldrick, SHELXL-97, University of Go
signals we have estimated pseudo-Ðrst-order rate constants at
various temperatures to secure linear Arrhenius plots from
Ž
ttingen, 1997.
New J. Chem., 1999, 23, 407È416
415

View Article Online
2
2
2
3
7 C. Bachmann, J. Demuynck and A. Veillard, J. Am. Chem. Soc.,
Brown, J. M. Williams and R. R. Schrock, J. Am. Chem. Soc., 1981,
103, 169.
32 R. N. Perutz and J. J. Turner, J. Am. Chem. Soc., 1975, 97, 4791;
Inorg. Chem., 1975, 14, 262; R. N. Perutz, Ph.D. Thesis, University
of Cambridge, 1974.
33 E. Rytter and D. M. Gruen, Spectrochim. Acta, Part A, 1979, 35,
199; D. R. Cowieson, A. J. Barnes and W. J. Orville-Thomas, J.
Raman Spectrosc., 1981, 10, 224.
34 B. McNamara, D. M. Becher, M. H. Towns and E. R. Grant, J.
Phys. Chem., 1994, 98, 4622.
35 J. C. Linehan, C. R. Yonker, J. T. Boys, S. T. Autrey, T. E. Bitter-
wolf and S. Gallagher, J. Am. Chem. Soc., 1998, 120, 5826.
1
978, 100, 2366.
8 E. Hirota, Y. Endo, S. Saito, K. Yoshida, I. Yamaguchi and K.
Machida, J. Mol. Spectrosc., 1981, 89, 223.
9 S. Yamamoto, M. Nakata, T. Fukuyama and K. Kuchitsu, J.
Phys. Chem., 1985, 89, 3298.
0 D. M. P. Mingos, in Comprehensive Organometallic Chemistry, ed.
G. Wilkinson, F. G. A. Stone and E. W. Abel, Pergamon, Oxford,
1
982, vol. 3, p. 1; S. D. Ittel and J. A. Ibers, Adv. Organomet.
Chem., 1976, 14, 33.
3
1 (a) R. A. Love, T. F. Koetzle, G. J. B. Williams, L. C. Andrews and
R. Bau, Inorg. Chem., 1975, 14, 2653; (b) J. A. K. Howard, J. L.
Spencer and S. A. Mason, Proc. R. Soc. L ondon A, 1983, 386, 145;
(
c) F.-M. Su, C. Cooper, S. J. Geib, A. L. Rheingold and J. M.
Mayer, J. Am. Chem. Soc., 1986, 108, 3545; (d) A. J. Schultz, R. K.
Paper 8/08621D
416
New J. Chem., 1999, 23, 407È416